the method I use in this case is:
- First explode the IFC geometry.In most cases they are a collection and I only want the Solids.
- To only get the Solids I use the GeometryFilter
- Next I use the SurfaceFootprintReplacer to get the footprint of each solid.
- As a last step you can use the Dissolver to merge all the areas together to a single feature.
Hope this helps.